Birefringence is one of the important features of light when propagating in an inhomogeneous medium crystal, and can be explained by the shear wave property of light. When light propagates in an inhomogeneous body (eg. a crystal other than those belonging to the cubic system), except for at a specific direction (along the direction of optical axis), it would change its vibration characteristics and is split into two polarized light beams having two electric field vectors of perpendicular vibration directions to each other, different propagation speeds and different refractive indexs. Such phenomenon is known as birefringence and such crystal is known as birefringent crystal. One of the two lights complies with the refraction law and is known as o-ray (ordinary ray), whose refractive index is represented by no; the other one does not comply with the refraction law and is known as e-ray (extraordinary ray), whose refractive index is represented by ne. These two lights are both polarized lights. Due to anisotropy of the crystalline material, the angle between the two refracted lights is associated to the propagation direction and polarization state of the light. The crystals causing birefringence can be divided into uniaxial and biaxial crystals. The material called uniaxial crystal belongs to trigonal, tetragonal or hexagonal systems, and the material called biaxial crystal belongs to triclinic, monoclinic or orthorhombic systems. The easy-to-use birefringent material is uniaxial crystal. The characteristics of the birefringent crystal can be utilized to obtain linear polarized light in order to achieve light beam shift, so that the birefringent crystals are key materials for producing optoisolator, circulator, beam shifter, optical polarizer, optical modulator and other optical components.
Common birefringent materials are calcite crystal, rutile crystal, LiNbO3 crystal, YVO4 crystal, α-BaB2O4 crystal and MgF2 crystal, etc. However, the calcite crystal mainly exists in the natural form and is difficult to synthetize. Ordinary calcite crystal can be only used in the waveband above 350 nm rather than the ultraviolet region. Besides, the general size of the calcite crystal is relatively small and the impurity content is relatively high, which cannot meet the requirements of large-size optical polarization components. Moreover, it is easy to cleave and difficult to process, resulting in poor crystal utilization. The rutile crystal also exists in the natural form and is difficult to synthetize. Moreover, the rutile crystal is relatively small in size and high in hardness and therefore is difficult to process. For LiNbO3, it is easy to get large-size crystals, but the birefringence index is too small. YVO4 is an excellent artificial birefringent crystal in performance, but its transmission range is 400-5,000 nm so it cannot be used in ultraviolet region. In addition, due to its high melting point, the YVO4 crystal must be grown in an iridium crucible in the manner of Czochral ski growth as well as at a weak oxygen atmosphere. Thus, there is a problem of valence change in iridium during the crystal growth, which results in a decline in crystal quality and makes it difficult to obtain high-quality crystals. α-BaB2O4 can easily crack during the crystal growth due to solid-state phase change. MgF2 crystal has a transmission range of 110-8,500 nm and is a potential candidate for application in deep ultraviolet region, but its birefringence index is too small to be suitable for Glan prism and can only be used to make Rochon prism. Moreover, it has a small walk-off angle, which will make the device size too large to be used. The Na3Ba2(B3O6)2F birefringent crystal provided in the present invention has not only a wide transmission range (175-3,350 nm) but also a large birefringence index (0.090-0.240), and can be used in deep ultraviolet waveband (175-350 nm).
In 2009, T. B. Bekker et al. (The Russian Federation) discovered Na3Ba2(B3O6)2F during exploration of growing β-BaB2O4 by the flux method and also reported the crystal structure: the crystal belonging to the hexagonal system, having a space group P63/m, with lattice parameters of a=7.3490(6) Å, c=12.6340(2) Å, V=590.93(12) Å3, Z=2. Meanwhile, the crystal was grown for 52 days by taking NaF or BaF2 as a flux and the relevant reports were published in Journal of Crystal Growth and other journals. However, there are no relevant reports about growth of the Na3Ba2(B3O6)2F crystal by the melt method or its birefringence index etc.
There are two major disadvantages for growing crystals by using the flux method: firstly, it takes a long cycle to grow the crystal, generally more than 30 days, so the growth efficiency is low; secondly, addition of the flux may introduce impurities into the crystal, which has negative effects on the optical quality of the crystal. Therefore, we employed the melt method to grow the Na3Ba2(B3O6)2F crystal according to its stoichiometric ratio without adding any flux to obtain the desired Na3Ba2(B3O6)2F crystal in 1-3 days, which is of high quality, without cracks or wrappages. Thus it can be seen that growing the crystal by the melt method not only greatly improves the growth efficiency but also ensure the optical quality, which is more suitable for large-scale growth of the Na3Ba2(B3O6)2F birefringent crystal.
By utilizing the resulting crystal, we characterized its refractive indexes for the first time and thereby obtained its birefringence indexes. We also characterized the transmission spectrum, based on which we proposed that Na3Ba2(B3O6)2F can be used as a birefringent crystal from infrared to deep ultraviolet wavebands and play an important role in the optical and communication fields. It could be used to produce optical polarization beam splitter prisms and infrared-deep ultraviolet optical communication components, including Glan prism, Wollaston prism, Rochon prism, beam splitting polarizer, as well as optoisolator, circulator, beam shifter, optical polarizer, optical modulator, optical polarizer, polarization beam splitter, phase retardation device, electro-optic modulation device and the like.